What are Condenser and Electret Microphones?
Condenser microphones make use of a thin piece of metal foil that is stretched above another piece of metal, as the foil moves it changes the capacitance of the system. This can be detected and turned into a current. Condenser mics are cheap and easy to make and have a good response.
Electret mics are similar to condenser mics in that they create a capacitance between two pieces of metal foil, but there is a polarizing voltage, or fixed-charge, applied to the back plate when it is made which is permanent. Electret microphones can be tiny and are used in cell phones.